The present invention relates to the field of electric motors, and, more particularly, to measuring a voltage or a current in a pulse width modulation (xe2x80x9cPWMxe2x80x9d) switch-driven winding.
In devices with PWM switch-driven windings such as static converters or induction machines, it may be necessary to precisely measure the instantaneous value of some significant electrical signal, such as the current or the back electromotive force (BEMF). Specifically, it may be necessary to distinguish the zero-cross events of these generally alternating signals. Detection of a zero current condition in an inductor of a converter is a fundamental requirement for switching on the power device when the current in the inductor has completely decayed. Moreover, the zero-cross event detection of back electromotive forces in the windings of induction motors may be useful to synchronize the phase switchings with the rotor""s position to produce the maximum and most uniform torque.
Detection of crossings of predetermined threshold values by alternating electrical signals may be accomplished by shifting the level of the signal by an amount equal to, and of a sign opposite of, the threshold value and detecting the zero-cross events of the level-shifted signal. Given that detection of the crossings of a certain threshold value by an electrical signal is equivalent to detecting the zero-cross events of a suitably level shifted signal, the ensuing description will refer only to the detection of zero-cross events for convenience.
Substantially, the detection of the zero-cross event of a BEMF or of the current flowing in a winding may be similarly carried out for both voltage and current signals. For example, the BEMF may be detected by a comparator coupled to the winding terminals. Furthermore, a zero-cross event of the current may be detected by converting the current signal into a voltage signal using a current sensing resistor in series with the winding and coupling a comparator to the terminals of the current sensing resistor. As will readily be appreciated by those of skill in the art, the problems that may be encountered using such approaches are generally the same and may be addressed in the same manner.
The method of the present invention is useful for detecting signals on PWM-driven windings, and may be implemented in a large number of different devices, such as DC-DC converters, induction machines, and brushless motors, for example. Given that correct reading of the BEMF zero-cross events in sensorless, brushless DC motors is important for correctly determining rotor position, by way of example only, the ensuing discussing will be directed to such a case.
A brushless motor includes a permanent magnet rotor and a stator. The stator includes a number of windings which may be connected in a star configuration, in a polygon (delta) configuration, or even independently from the one another, for example. Star or polygon configurations may have a number of externally accessible terminals equal to the number of phase windings of the motor (and with an additional terminal if the star center is made accessible). In the case of motors with independent phase windings, both terminals of all the phase windings are accessible.
Electronically switched DC motors are widely used in so-called xe2x80x9chardxe2x80x9d disk, compact disk (xe2x80x9cCDxe2x80x9d), and digital video disk (xe2x80x9cDVDxe2x80x9d) drives, and as tape drive motors in video recorders, for example. The driving and control system of a three-phase motor may be an integrated circuit that includes a power stage for driving the phase windings and either a full-wave three-phase half-bridge circuit (in the case of star or triangle connected motors) or three full-bridge circuits (in the case of motors with independently driven windings).
The most common driving mode is referred to as xe2x80x9cbipolar,xe2x80x9d which is where two windings are driven while the third is inactive (e.g., the output of the relative bridge or half-bridge is in a state of high impedance or tristate). The phases are switched according to a cyclic sequence, synchronized with the rotor""s instantaneous position. In a bipolar driving system, the instantaneous position may be detected by monitoring the BEMF in the phase winding (which in the current phase is tristate) or by Hall effect sensors (an approach seldom used due to its higher cost). In the case of monitoring the BEMF, it is a common practice to detect the zero-cross events of its periodic waveform.
European patent applications Nos. 96830440.2 and 98202320.2, also assigned to the present assignee, describe different techniques for synchronizing the phase switchings. The comparators that are normally used to monitor the BEMF in question may be gravely affected by the switching noise. In order to prevent the generation of spurious switchings of the outputs of the comparators, masking the outputs of the comparators is typically performed. For example, a suitable masking circuit may be realized with an AND gate having as its inputs the output of the voltage comparator and a masking pulse or signal. This prevents the propagation of a zero-cross event detection signal for a time interval (defined by the duration of the masking pulse) after each phase switching.
It is also common to filter the high frequency noise introduced by the PWM signal on the BEMF and thereafter compare the filtered value with the zero value to enhance the precision of the zero-cross event detection. Yet, this approach introduces phase errors and, in some cases, undesired non-systematic errors. Such errors increase the jitter of the speed of the motor.
It is therefore an object of the present invention to provide a method for detecting the zero-cross events of an induced voltage or of a current in a PWM driven winding with greater precision.
This and other objects, features, and advantages in accordance with the present invention are provided by a method for detecting the zero-cross events of an induced voltage or of a current in a winding driven by a PWM signal. The method may include storing a time between a last two zero-cross events and synchronizing the PWM driving signal from a last zero-cross event having a duration equal to a difference between an established time based upon the stored time interval and a first time. If a new zero-cross event is not detected within the established time, switching of the PWM driving signal may be disabled for a time having a maximum duration equal to a second time or until a new zero-cross event is detected.
In one embodiment of the present invention, the established time may be equal to the stored time, for example. While this embodiment is simple to implement, it may be problematic in certain applications due to its sensitivity to variations of the zero-cross events. Moreover, in the case of PWM-driven motors, fabrication asymmetries and other factors may cause spurious sign variations in the succession of zero-cross events.
In a alternate embodiment of the invention, the established time is equal to an arithmetic mean of a plurality of times between consecutive zero-cross events. The plurality of times may be stored in a shift register configured in a first-in-first-out (FIFO) mode. In this way, the established time is substantially free from stochastic errors. In yet another embodiment that is particularly effective in driving motors, the established time may be calculated by applying a linear prediction (for example, a linear regression) or a non-linear (for example, a polynomial regression) method to the plurality of times. Accordingly, discrimination between the different phases of motor function (i.e., acceleration, constant speed, and deceleration) and accurate estimations of the occurrence of threshold crossings are achieved.
Furthermore, the first time interval may be chosen so that the zero-cross events will occur during a second half of a time interval between two consecutive fronts of the PWM driving signal. It is thereby possible in brushless DC sensorless motors, for example, to achieve a significant reduction of motor speed jitter from causes such as fabrication asymmetries of the motor and the like.
A system for driving a polyphase motor based upon zero-cross events of an induced back electromotive force (BEMF) or of the nullification instant of a periodic current may include a comparator of the BEMF or of the nullification instant of periodic current and producing a first logic signal at each zero-cross or null current event, a masking circuit for the first logic signal controlled by a second logic signal and producing a third logic signal, a generator for generating a PWM driving signal and the second logic signal, and a controller monitoring the third logic signal and controlling the generator. The controller may store a time between a last two zero-cross events and synchronize the PWM driving signal from a last zero-cross event having a duration equal to a difference between an established time based upon the stored time interval and a first time. If a new zero-cross event is not detected within the established time, the controller may disable switching of the PWM driving signal for a time having a maximum duration equal to a second time or until a new zero-cross event is detected.